Network Working Group R. Housley
Request for Comments: 5652 Vigil Security
Obsoletes: 3852 September 2009
Category: Standards Track
Cryptographic Message Syntax (CMS)
Abstract
This document describes the Cryptographic Message Syntax (CMS). This
syntax is used to digitally sign, digest, authenticate, or encrypt
arbitrary message content.
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright and License Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Housley Standards Track [Page 1]
RFC 5652 Cryptographic Message Syntax September 2009
Table of Contents
1. Introduction ....................................................3
1.1. Evolution of the CMS .......................................4
1.1.1. Changes Since PKCS #7 Version 1.5 ...................4
1.1.2. Changes Since RFC 2630 ..............................4
1.1.3. Changes Since RFC 3369 ..............................5
1.1.4. Changes Since RFC 3852 ..............................5
1.2. Terminology ................................................5
1.3. Version Numbers ............................................6
2. General Overview ................................................6
3. General Syntax ..................................................7
4. Data Content Type ...............................................7
5. Signed-data Content Type ........................................8
5.1. SignedData Type ............................................9
5.2. EncapsulatedContentInfo Type ..............................11
5.2.1. Compatibility with PKCS #7 .........................12
5.3. SignerInfo Type ...........................................13
5.4. Message Digest Calculation Process ........................16
5.5. Signature Generation Process ..............................16
5.6. Signature Verification Process ............................17
6. Enveloped-Data Content Type ....................................17
6.1. EnvelopedData Type ........................................18
6.2. RecipientInfo Type ........................................21
6.2.1. KeyTransRecipientInfo Type .........................22
6.2.2. KeyAgreeRecipientInfo Type .........................23
6.2.3. KEKRecipientInfo Type ..............................25
6.2.4. PasswordRecipientInfo Type .........................26
6.2.5. OtherRecipientInfo Type ............................27
6.3. Content-encryption Process ................................27
6.4. Key-Encryption Process ....................................28
7. Digested-Data Content Type .....................................28
8. Encrypted-Data Content Type ....................................29
9. Authenticated-Data Content Type ................................30
9.1. AuthenticatedData Type ....................................31
9.2. MAC Generation ............................................33
9.3. MAC Verification ..........................................34
10. Useful Types ..................................................34
10.1. Algorithm Identifier Types ...............................35
10.1.1. DigestAlgorithmIdentifier .........................35
10.1.2. SignatureAlgorithmIdentifier ......................35
10.1.3. KeyEncryptionAlgorithmIdentifier ..................35
10.1.4. ContentEncryptionAlgorithmIdentifier ..............36
10.1.5. MessageAuthenticationCodeAlgorithm ................36
10.1.6. KeyDerivationAlgorithmIdentifier ..................36
10.2. Other Useful Types .......................................36
10.2.1. RevocationInfoChoices .............................36
10.2.2. CertificateChoices ................................37
Housley Standards Track [Page 2]
RFC 5652 Cryptographic Message Syntax September 2009
10.2.3. CertificateSet ....................................38
10.2.4. IssuerAndSerialNumber .............................38
10.2.5. CMSVersion ........................................39
10.2.6. UserKeyingMaterial ................................39
10.2.7. OtherKeyAttribute .................................39
11. Useful Attributes .............................................39
11.1. Content Type .............................................40
11.2. Message Digest ...........................................40
11.3. Signing Time .............................................41
11.4. Countersignature .........................................42
12. ASN.1 Modules .................................................43
12.1. CMS ASN.1 Module .........................................44
12.2. Version 1 Attribute Certificate ASN.1 Module .............51
13. References ....................................................52
13.1. Normative References .....................................52
13.2. Informative References ...................................53
14. Security Considerations .......................................54
15. Acknowledgments ...............................................56
1. Introduction
This document describes the Cryptographic Message Syntax (CMS). This
syntax is used to digitally sign, digest, authenticate, or encrypt
arbitrary message content.
The CMS describes an encapsulation syntax for data protection. It
supports digital signatures and encryption. The syntax allows
multiple encapsulations; one encapsulation envelope can be nested
inside another. Likewise, one party can digitally sign some
previously encapsulated data. It also allows arbitrary attributes,
such as signing time, to be signed along with the message content,
and it provides for other attributes such as countersignatures to be
associated with a signature.
The CMS can support a variety of architectures for certificate-based
key management, such as the one defined by the PKIX (Public Key
Infrastructure using X.509) working group [PROFILE].
The CMS values are generated using ASN.1 [X.208-88], using BER-
encoding (Basic Encoding Rules) [X.209-88]. Values are typically
represented as octet strings. While many systems are capable of
transmitting arbitrary octet strings reliably, it is well known that
many electronic mail systems are not. This document does not address
mechanisms for encoding octet strings for reliable transmission in
such environments.
Housley Standards Track [Page 3]
RFC 5652 Cryptographic Message Syntax September 2009
1.1. Evolution of the CMS
The CMS is derived from PKCS #7 version 1.5, which is documented in
RFC 2315 [PKCS#7]. PKCS #7 version 1.5 was developed outside of the
IETF; it was originally published as an RSA Laboratories Technical
Note in November 1993. Since that time, the IETF has taken
responsibility for the development and maintenance of the CMS.
Today, several important IETF Standards-Track protocols make use of
the CMS.
This section describes that changes that the IETF has made to the CMS
in each of the published versions.
1.1.1. Changes Since PKCS #7 Version 1.5
RFC 2630 [CMS1] was the first version of the CMS on the IETF
Standards Track. Wherever possible, backward compatibility with PKCS
#7 version 1.5 is preserved; however, changes were made to
accommodate version 1 attribute certificate transfer and to support
algorithm-independent key management. PKCS #7 version 1.5 included
support only for key transport. RFC 2630 adds support for key
agreement and previously distributed symmetric key-encryption key
techniques.
1.1.2. Changes Since RFC 2630
RFC 3369 [CMS2] obsoletes RFC 2630 [CMS1] and RFC 3211 [PWRI].
Password-based key management is included in the CMS specification,
and an extension mechanism to support new key management schemes
without further changes to the CMS is specified. Backward
compatibility with RFC 2630 and RFC 3211 is preserved; however,
version 2 attribute certificate transfer is added, and the use of
version 1 attribute certificates is deprecated.
Secure/Multipurpose Internet Mail Extensions (S/MIME) v2 signatures
[MSG2], which are based on PKCS #7 version 1.5, are compatible with
S/MIME v3 signatures [MSG3]and S/MIME v3.1 signatures [MSG3.1].
However, there are some subtle compatibility issues with signatures
based on PKCS #7 version 1.5. These issues are discussed in Section
5.2.1. These issues remain with the current version of the CMS.
Specific cryptographic algorithms are not discussed in this document,
but they were discussed in RFC 2630. The discussion of specific
cryptographic algorithms has been moved to a separate document
[CMSALG]. Separation of the protocol and algorithm specifications
allows the IETF to update each document independently. This
specification does not require the implementation of any particular
Housley Standards Track [Page 4]
RFC 5652 Cryptographic Message Syntax September 2009
algorithms. Rather, protocols that rely on the CMS are expected to
choose appropriate algorithms for their environment. The algorithms
may be selected from [CMSALG] or elsewhere.
1.1.3. Changes Since RFC 3369
RFC 3852 [CMS3] obsoletes RFC 3369 [CMS2]. As discussed in the
previous section, RFC 3369 introduced an extension mechanism to
support new key management schemes without further changes to the
CMS. RFC 3852 introduces a similar extension mechanism to support
additional certificate formats and revocation status information
formats without further changes to the CMS. These extensions are
primarily documented in Sections 10.2.1 and 10.2.2. Backward
compatibility with earlier versions of the CMS is preserved.
The use of version numbers is described in Section 1.3.
Since the publication of RFC 3369, a few errata have been noted.
These errata are posted on the RFC Editor web site. These errors
have been corrected in this document.
The text in Section 11.4 that describes the counter signature
unsigned attribute is clarified. Hopefully, the revised text is
clearer about the portion of the SignerInfo signature that is covered
by a countersignature.
1.1.4. Changes Since RFC 3852
This document obsoletes RFC 3852 [CMS3]. The primary reason for the
publication of this document is to advance the CMS along the
standards maturity ladder.
This document includes the clarifications that were originally
published in RFC 4853 [CMSMSIG] regarding the proper handling of the
SignedData protected content type when more than one digital
signature is present.
Since the publication of RFC 3852, a few errata have been noted.
These errata are posted on the RFC Editor web site. These errors
have been corrected in this document.
1.2. Terminology
In this document, the key words MUST, MUST NOT, REQUIRED, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL are to be interpreted as
described in [STDWORDS].
Housley Standards Track [Page 5]
RFC 5652 Cryptographic Message Syntax September 2009
1.3. Version Numbers
Each of the major data structures includes a version number as the
first item in the data structure. The version numbers are intended
to avoid ASN.1 decode errors. Some implementations do not check the
version number prior to attempting a decode, and if a decode error
occurs, then the version number is checked as part of the error
handling routine. This is a reasonable approach; it places error
processing outside of the fast path. This approach is also forgiving
when an incorrect version number is used by the sender.
Most of the initial version numbers were assigned in PKCS #7 version
1.5. Others were assigned when the structure was initially created.
Whenever a structure is updated, a higher version number is assigned.
However, to ensure maximum interoperability, the higher version
number is only used when the new syntax feature is employed. That
is, the lowest version number that supports the generated syntax is
used.
2. General Overview
The CMS is general enough to support many different content types.
This document defines one protection content, ContentInfo.
ContentInfo encapsulates a single identified content type, and the
identified type may provide further encapsulation. This document
defines six content types: data, signed-data, enveloped-data,
digested-data, encrypted-data, and authenticated-data. Additional
content types can be defined outside this document.
An implementation that conforms to this specification MUST implement
the protection content, ContentInfo, and MUST implement the data,
signed-data, and enveloped-data content types. The other content
types MAY be implemented.
As a general design philosophy, each content type permits single pass
processing using indefinite-length Basic Encoding Rules (BER)
encoding. Single-pass operation is especially helpful if content is
large, stored on tapes, or is "piped" from another process. Single-
pass operation has one significant drawback: it is difficult to
perform encode operations using the Distinguished Encoding Rules
(DER) [X.509-88] encoding in a single pass since the lengths of the
various components may not be known in advance. However, signed
attributes within the signed-data content type and authenticated
attributes within the authenticated-data content type need to be
transmitted in DER form to ensure that recipients can verify a
content that contains one or more unrecognized attributes. Signed
attributes and authenticated attributes are the only data types used
in the CMS that require DER encoding.
Housley Standards Track [Page 6]
RFC 5652 Cryptographic Message Syntax September 2009
3. General Syntax
The following object identifier identifies the content information
type:
id-ct-contentInfo OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) ct(1) 6 }
The CMS associates a content type identifier with a content. The
syntax MUST have ASN.1 type ContentInfo:
ContentInfo ::= SEQUENCE {
contentType ContentType,
content [0] EXPLICIT ANY DEFINED BY contentType }
ContentType ::= OBJECT IDENTIFIER
The fields of ContentInfo have the following meanings:
contentType indicates the type of the associated content. It is
an object identifier; it is a unique string of integers assigned
by an authority that defines the content type.
content is the associated content. The type of content can be
determined uniquely by contentType. Content types for data,
signed-data, enveloped-data, digested-data, encrypted-data, and
authenticated-data are defined in this document. If additional
content types are defined in other documents, the ASN.1 type
defined SHOULD NOT be a CHOICE type.
4. Data Content Type
The following object identifier identifies the data content type:
id-data OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 1 }
The data content type is intended to refer to arbitrary octet
strings, such as ASCII text files; the interpretation is left to the
application. Such strings need not have any internal structure
(although they could have their own ASN.1 definition or other
structure).
S/MIME uses id-data to identify MIME-encoded content. The use of
this content identifier is specified in RFC 2311 for S/MIME v2
[MSG2], RFC 2633 for S/MIME v3 [MSG3], and RFC 3851 for S/MIME v3.1
[MSG3.1].
Housley Standards Track [Page 7]
RFC 5652 Cryptographic Message Syntax September 2009
The data content type is generally encapsulated in the signed-data,
enveloped-data, digested-data, encrypted-data, or authenticated-data
content type.
5. Signed-data Content Type
The signed-data content type consists of a content of any type and
zero or more signature values. Any number of signers in parallel can
sign any type of content.
The typical application of the signed-data content type represents
one signer's digital signature on content of the data content type.
Another typical application disseminates certificates and certificate
revocation lists (CRLs).
The process by which signed-data is constructed involves the
following steps:
1. For each signer, a message digest, or hash value, is computed on
the content with a signer-specific message-digest algorithm. If
the signer is signing any information other than the content, the
message digest of the content and the other information are
digested with the signer's message digest algorithm (see Section
5.4), and the result becomes the "message digest."
2. For each signer, the message digest is digitally signed using the
signer's private key.
3. For each signer, the signature value and other signer-specific
information are collected into a SignerInfo value, as defined in
Section 5.3. Certificates and CRLs for each signer, and those
not corresponding to any signer, are collected in this step.
4. The message digest algorithms for all the signers and the
SignerInfo values for all the signers are collected together with
the content into a SignedData value, as defined in Section 5.1.
A recipient independently computes the message digest. This message
digest and the signer's public key are used to verify the signature
value. The signer's public key is referenced in one of two ways. It
can be referenced by an issuer distinguished name along with an
issuer-specific serial number to uniquely identify the certificate
that contains the public key. Alternatively, it can be referenced by
a subject key identifier, which accommodates both certified and
uncertified public keys. While not required, the signer's
certificate can be included in the SignedData certificates field.
Housley Standards Track [Page 8]
RFC 5652 Cryptographic Message Syntax September 2009
When more than one signature is present, the successful validation of
one signature associated with a given signer is usually treated as a
successful signature by that signer. However, there are some
application environments where other rules are needed. An
application that employs a rule other than one valid signature for
each signer must specify those rules. Also, where simple matching of
the signer identifier is not sufficient to determine whether the
signatures were generated by the same signer, the application
specification must describe how to determine which signatures were
generated by the same signer. Support of different communities of
recipients is the primary reason that signers choose to include more
than one signature. For example, the signed-data content type might
include signatures generated with the RSA signature algorithm and
with the Elliptic Curve Digital Signature Algorithm (ECDSA) signature
algorithm. This allows recipients to verify the signature associated
with one algorithm or the other.
This section is divided into six parts. The first part describes the
top-level type SignedData, the second part describes
EncapsulatedContentInfo, the third part describes the per-signer
information type SignerInfo, and the fourth, fifth, and sixth parts
describe the message digest calculation, signature generation, and
signature verification processes, respectively.
5.1. SignedData Type
The following object identifier identifies the signed-data content
type:
id-signedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 2 }
The signed-data content type shall have ASN.1 type SignedData:
SignedData ::= SEQUENCE {
version CMSVersion,
digestAlgorithms DigestAlgorithmIdentifiers,
encapContentInfo EncapsulatedContentInfo,
certificates [0] IMPLICIT CertificateSet OPTIONAL,
crls [1] IMPLICIT RevocationInfoChoices OPTIONAL,
signerInfos SignerInfos }
DigestAlgorithmIdentifiers ::= SET OF DigestAlgorithmIdentifier
SignerInfos ::= SET OF SignerInfo
Housley Standards Track [Page 9]
RFC 5652 Cryptographic Message Syntax September 2009
The fields of type SignedData have the following meanings:
version is the syntax version number. The appropriate value
depends on certificates, eContentType, and SignerInfo. The
version MUST be assigned as follows:
IF ((certificates is present) AND
(any certificates with a type of other are present)) OR
((crls is present) AND
(any crls with a type of other are present))
THEN version MUST be 5
ELSE
IF (certificates is present) AND
(any version 2 attribute certificates are present)
THEN version MUST be 4
ELSE
IF ((certificates is present) AND
(any version 1 attribute certificates are present)) OR
(any SignerInfo structures are version 3) OR
(encapContentInfo eContentType is other than id-data)
THEN version MUST be 3
ELSE version MUST be 1
digestAlgorithms is a collection of message digest algorithm
identifiers. There MAY be any number of elements in the
collection, including zero. Each element identifies the message
digest algorithm, along with any associated parameters, used by
one or more signer. The collection is intended to list the
message digest algorithms employed by all of the signers, in any
order, to facilitate one-pass signature verification.
Implementations MAY fail to validate signatures that use a digest
algorithm that is not included in this set. The message digesting
process is described in Section 5.4.
encapContentInfo is the signed content, consisting of a content
type identifier and the content itself. Details of the
EncapsulatedContentInfo type are discussed in Section 5.2.
certificates is a collection of certificates. It is intended that
the set of certificates be sufficient to contain certification
paths from a recognized "root" or "top-level certification
authority" to all of the signers in the signerInfos field. There
may be more certificates than necessary, and there may be
certificates sufficient to contain certification paths from two or
more independent top-level certification authorities. There may
also be fewer certificates than necessary, if it is expected that
recipients have an alternate means of obtaining necessary
Housley Standards Track [Page 10]
RFC 5652 Cryptographic Message Syntax September 2009
certificates (e.g., from a previous set of certificates). The
signer's certificate MAY be included. The use of version 1
attribute certificates is strongly discouraged.
crls is a collection of revocation status information. It is
intended that the collection contain information sufficient to
determine whether the certificates in the certificates field are
valid, but such correspondence is not necessary. Certificate
revocation lists (CRLs) are the primary source of revocation
status information. There MAY be more CRLs than necessary, and
there MAY also be fewer CRLs than necessary.
signerInfos is a collection of per-signer information. There MAY
be any number of elements in the collection, including zero. When
the collection represents more than one signature, the successful
validation of one of signature from a given signer ought to be
treated as a successful signature by that signer. However, there
are some application environments where other rules are needed.
The details of the SignerInfo type are discussed in Section 5.3.
Since each signer can employ a different digital signature
technique, and future specifications could update the syntax, all
implementations MUST gracefully handle unimplemented versions of
SignerInfo. Further, since all implementations will not support
every possible signature algorithm, all implementations MUST
gracefully handle unimplemented signature algorithms when they are
encountered.
5.2. EncapsulatedContentInfo Type
The content is represented in the type EncapsulatedContentInfo:
EncapsulatedContentInfo ::= SEQUENCE {
eContentType ContentType,
eContent [0] EXPLICIT OCTET STRING OPTIONAL }
ContentType ::= OBJECT IDENTIFIER
The fields of type EncapsulatedContentInfo have the following
meanings:
eContentType is an object identifier. The object identifier
uniquely specifies the content type.
eContent is the content itself, carried as an octet string. The
eContent need not be DER encoded.
Housley Standards Track [Page 11]
RFC 5652 Cryptographic Message Syntax September 2009
The optional omission of the eContent within the
EncapsulatedContentInfo field makes it possible to construct
"external signatures". In the case of external signatures, the
content being signed is absent from the EncapsulatedContentInfo value
included in the signed-data content type. If the eContent value
within EncapsulatedContentInfo is absent, then the signatureValue is
calculated and the eContentType is assigned as though the eContent
value was present.
In the degenerate case where there are no signers, the
EncapsulatedContentInfo value being "signed" is irrelevant. In this
case, the content type within the EncapsulatedContentInfo value being
"signed" MUST be id-data (as defined in Section 4), and the content
field of the EncapsulatedContentInfo value MUST be omitted.
5.2.1. Compatibility with PKCS #7
This section contains a word of warning to implementers that wish to
support both the CMS and PKCS #7 [PKCS#7] SignedData content types.
Both the CMS and PKCS #7 identify the type of the encapsulated
content with an object identifier, but the ASN.1 type of the content
itself is variable in PKCS #7 SignedData content type.
PKCS #7 defines content as:
content [0] EXPLICIT ANY DEFINED BY contentType OPTIONAL
The CMS defines eContent as:
eContent [0] EXPLICIT OCTET STRING OPTIONAL
The CMS definition is much easier to use in most applications, and it
is compatible with both S/MIME v2 and S/MIME v3. S/MIME signed
messages using the CMS and PKCS #7 are compatible because identical
signed message formats are specified in RFC 2311 for S/MIME v2
[MSG2], RFC 2633 for S/MIME v3 [MSG3], and RFC 3851 for S/MIME v3.1
[MSG3.1]. S/MIME v2 encapsulates the MIME content in a Data type
(that is, an OCTET STRING) carried in the SignedData contentInfo
content ANY field, and S/MIME v3 carries the MIME content in the
SignedData encapContentInfo eContent OCTET STRING. Therefore, in
S/MIME v2, S/MIME v3, and S/MIME v3.1, the MIME content is placed in
an OCTET STRING and the message digest is computed over the identical
portions of the content. That is, the message digest is computed
over the octets comprising the value of the OCTET STRING, neither the
tag nor length octets are included.
Housley Standards Track [Page 12]
RFC 5652 Cryptographic Message Syntax September 2009
There are incompatibilities between the CMS and PKCS #7 SignedData
types when the encapsulated content is not formatted using the Data
type. For example, when an RFC 2634 signed receipt [ESS] is
encapsulated in the CMS SignedData type, then the Receipt SEQUENCE is
encoded in the SignedData encapContentInfo eContent OCTET STRING and
the message digest is computed using the entire Receipt SEQUENCE
encoding (including tag, length and value octets). However, if an
RFC 2634 signed receipt is encapsulated in the PKCS #7 SignedData
type, then the Receipt SEQUENCE is DER encoded [X.509-88] in the
SignedData contentInfo content ANY field (a SEQUENCE, not an OCTET
STRING). Therefore, the message digest is computed using only the
value octets of the Receipt SEQUENCE encoding.
The following strategy can be used to achieve backward compatibility
with PKCS #7 when processing SignedData content types. If the
implementation is unable to ASN.1 decode the SignedData type using
the CMS SignedData encapContentInfo eContent OCTET STRING syntax,
then the implementation MAY attempt to decode the SignedData type
using the PKCS #7 SignedData contentInfo content ANY syntax and
compute the message digest accordingly.
The following strategy can be used to achieve backward compatibility
with PKCS #7 when creating a SignedData content type in which the
encapsulated content is not formatted using the Data type.
Implementations MAY examine the value of the eContentType, and then
adjust the expected DER encoding of eContent based on the object
identifier value. For example, to support Microsoft Authenticode
[MSAC], the following information MAY be included:
eContentType Object Identifier is set to { 1 3 6 1 4 1 311 2 1 4 }
eContent contains DER-encoded Authenticode signing information
5.3. SignerInfo Type
Per-signer information is represented in the type SignerInfo:
SignerInfo ::= SEQUENCE {
version CMSVersion,
sid SignerIdentifier,
digestAlgorithm DigestAlgorithmIdentifier,
signedAttrs [0] IMPLICIT SignedAttributes OPTIONAL,
signatureAlgorithm SignatureAlgorithmIdentifier,
signature SignatureValue,
unsignedAttrs [1] IMPLICIT UnsignedAttributes OPTIONAL }
Housley Standards Track [Page 13]
RFC 5652 Cryptographic Message Syntax September 2009
SignerIdentifier ::= CHOICE {
issuerAndSerialNumber IssuerAndSerialNumber,
subjectKeyIdentifier [0] SubjectKeyIdentifier }
SignedAttributes ::= SET SIZE (1..MAX) OF Attribute
UnsignedAttributes ::= SET SIZE (1..MAX) OF Attribute
Attribute ::= SEQUENCE {
attrType OBJECT IDENTIFIER,
attrValues SET OF AttributeValue }
AttributeValue ::= ANY
SignatureValue ::= OCTET STRING
The fields of type SignerInfo have the following meanings:
version is the syntax version number. If the SignerIdentifier is
the CHOICE issuerAndSerialNumber, then the version MUST be 1. If
the SignerIdentifier is subjectKeyIdentifier, then the version
MUST be 3.
sid specifies the signer's certificate (and thereby the signer's
public key). The signer's public key is needed by the recipient
to verify the signature. SignerIdentifier provides two
alternatives for specifying the signer's public key. The
issuerAndSerialNumber alternative identifies the signer's
certificate by the issuer's distinguished name and the certificate
serial number; the subjectKeyIdentifier identifies the signer's
certificate by a key identifier. When an X.509 certificate is
referenced, the key identifier matches the X.509
subjectKeyIdentifier extension value. When other certificate
formats are referenced, the documents that specify the certificate
format and their use with the CMS must include details on matching
the key identifier to the appropriate certificate field.
Implementations MUST support the reception of the
issuerAndSerialNumber and subjectKeyIdentifier forms of
SignerIdentifier. When generating a SignerIdentifier,
implementations MAY support one of the forms (either
issuerAndSerialNumber or subjectKeyIdentifier) and always use it,
or implementations MAY arbitrarily mix the two forms. However,
subjectKeyIdentifier MUST be used to refer to a public key
contained in a non-X.509 certificate.
digestAlgorithm identifies the message digest algorithm, and any
associated parameters, used by the signer. The message digest is
computed on either the content being signed or the content
Housley Standards Track [Page 14]
RFC 5652 Cryptographic Message Syntax September 2009
together with the signed attributes using the process described in
Section 5.4. The message digest algorithm SHOULD be among those
listed in the digestAlgorithms field of the associated SignerData.
Implementations MAY fail to validate signatures that use a digest
algorithm that is not included in the SignedData digestAlgorithms
set.
signedAttrs is a collection of attributes that are signed. The
field is optional, but it MUST be present if the content type of
the EncapsulatedContentInfo value being signed is not id-data.
SignedAttributes MUST be DER encoded, even if the rest of the
structure is BER encoded. Useful attribute types, such as signing
time, are defined in Section 11. If the field is present, it MUST
contain, at a minimum, the following two attributes:
A content-type attribute having as its value the content type
of the EncapsulatedContentInfo value being signed. Section
11.1 defines the content-type attribute. However, the
content-type attribute MUST NOT be used as part of a
countersignature unsigned attribute as defined in Section 11.4.
A message-digest attribute, having as its value the message
digest of the content. Section 11.2 defines the message-digest
attribute.
signatureAlgorithm identifies the signature algorithm, and any
associated parameters, used by the signer to generate the digital
signature.
signature is the result of digital signature generation, using the
message digest and the signer's private key. The details of the
signature depend on the signature algorithm employed.
unsignedAttrs is a collection of attributes that are not signed.
The field is optional. Useful attribute types, such as
countersignatures, are defined in Section 11.
The fields of type SignedAttribute and UnsignedAttribute have the
following meanings:
attrType indicates the type of attribute. It is an object
identifier.
attrValues is a set of values that comprise the attribute. The
type of each value in the set can be determined uniquely by
attrType. The attrType can impose restrictions on the number of
items in the set.
Housley Standards Track [Page 15]
RFC 5652 Cryptographic Message Syntax September 2009
5.4. Message Digest Calculation Process
The message digest calculation process computes a message digest on
either the content being signed or the content together with the
signed attributes. In either case, the initial input to the message
digest calculation process is the "value" of the encapsulated content
being signed. Specifically, the initial input is the
encapContentInfo eContent OCTET STRING to which the signing process
is applied. Only the octets comprising the value of the eContent
OCTET STRING are input to the message digest algorithm, not the tag
or the length octets.
The result of the message digest calculation process depends on
whether the signedAttrs field is present. When the field is absent,
the result is just the message digest of the content as described
above. When the field is present, however, the result is the message
digest of the complete DER encoding of the SignedAttrs value
contained in the signedAttrs field. Since the SignedAttrs value,
when present, must contain the content-type and the message-digest
attributes, those values are indirectly included in the result. The
content-type attribute MUST NOT be included in a countersignature
unsigned attribute as defined in Section 11.4. A separate encoding
of the signedAttrs field is performed for message digest calculation.
The IMPLICIT [0] tag in the signedAttrs is not used for the DER
encoding, rather an EXPLICIT SET OF tag is used. That is, the DER
encoding of the EXPLICIT SET OF tag, rather than of the IMPLICIT [0]
tag, MUST be included in the message digest calculation along with
the length and content octets of the SignedAttributes value.
When the signedAttrs field is absent, only the octets comprising the
value of the SignedData encapContentInfo eContent OCTET STRING (e.g.,
the contents of a file) are input to the message digest calculation.
This has the advantage that the length of the content being signed
need not be known in advance of the signature generation process.
Although the encapContentInfo eContent OCTET STRING tag and length
octets are not included in the message digest calculation, they are
protected by other means. The length octets are protected by the
nature of the message digest algorithm since it is computationally
infeasible to find any two distinct message contents of any length
that have the same message digest.
5.5. Signature Generation Process
The input to the signature generation process includes the result of
the message digest calculation process and the signer's private key.
The details of the signature generation depend on the signature
algorithm employed. The object identifier, along with any
Housley Standards Track [Page 16]
RFC 5652 Cryptographic Message Syntax September 2009
parameters, that specifies the signature algorithm employed by the
signer is carried in the signatureAlgorithm field. The signature
value generated by the signer MUST be encoded as an OCTET STRING and
carried in the signature field.
5.6. Signature Verification Process
The input to the signature verification process includes the result
of the message digest calculation process and the signer's public
key. The recipient MAY obtain the correct public key for the signer
by any means, but the preferred method is from a certificate obtained
from the SignedData certificates field. The selection and validation
of the signer's public key MAY be based on certification path
validation (see [PROFILE]) as well as other external context, but is
beyond the scope of this document. The details of the signature
verification depend on the signature algorithm employed.
The recipient MUST NOT rely on any message digest values computed by
the originator. If the SignedData signerInfo includes
signedAttributes, then the content message digest MUST be calculated
as described in Section 5.4. For the signature to be valid, the
message digest value calculated by the recipient MUST be the same as
the value of the messageDigest attribute included in the
signedAttributes of the SignedData signerInfo.
If the SignedData signerInfo includes signedAttributes, then the
content-type attribute value MUST match the SignedData
encapContentInfo eContentType value.
6. Enveloped-data Content Type
The enveloped-data content type consists of an encrypted content of
any type and encrypted content-encryption keys for one or more
recipients. The combination of the encrypted content and one
encrypted content-encryption key for a recipient is a "digital
envelope" for that recipient. Any type of content can be enveloped
for an arbitrary number of recipients using any of the supported key
management techniques for each recipient.
The typical application of the enveloped-data content type will
represent one or more recipients' digital envelopes on content of the
data or signed-data content types.
Enveloped-data is constructed by the following steps:
1. A content-encryption key for a particular content-encryption
algorithm is generated at random.
Housley Standards Track [Page 17]
RFC 5652 Cryptographic Message Syntax September 2009
2. The content-encryption key is encrypted for each recipient. The
details of this encryption depend on the key management algorithm
used, but four general techniques are supported:
key transport: the content-encryption key is encrypted in the
recipient's public key;
key agreement: the recipient's public key and the sender's
private key are used to generate a pairwise symmetric key, then
the content-encryption key is encrypted in the pairwise
symmetric key;
symmetric key-encryption keys: the content-encryption key is
encrypted in a previously distributed symmetric key-encryption
key; and
passwords: the content-encryption key is encrypted in a key-
encryption key that is derived from a password or other shared
secret value.
3. For each recipient, the encrypted content-encryption key and
other recipient-specific information are collected into a
RecipientInfo value, defined in Section 6.2.
4. The content is encrypted with the content-encryption key.
Content encryption may require that the content be padded to a
multiple of some block size; see Section 6.3.
5. The RecipientInfo values for all the recipients are collected
together with the encrypted content to form an EnvelopedData
value as defined in Section 6.1.
A recipient opens the digital envelope by decrypting one of the
encrypted content-encryption keys and then decrypting the encrypted
content with the recovered content-encryption key.
This section is divided into four parts. The first part describes
the top-level type EnvelopedData, the second part describes the per-
recipient information type RecipientInfo, and the third and fourth
parts describe the content-encryption and key-encryption processes.
6.1. EnvelopedData Type
The following object identifier identifies the enveloped-data content
type:
id-envelopedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 3 }
Housley Standards Track [Page 18]
RFC 5652 Cryptographic Message Syntax September 2009
The enveloped-data content type shall have ASN.1 type EnvelopedData:
EnvelopedData ::= SEQUENCE {
version CMSVersion,
originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL,
recipientInfos RecipientInfos,
encryptedContentInfo EncryptedContentInfo,
unprotectedAttrs [1] IMPLICIT UnprotectedAttributes OPTIONAL }
OriginatorInfo ::= SEQUENCE {
certs [0] IMPLICIT CertificateSet OPTIONAL,
crls [1] IMPLICIT RevocationInfoChoices OPTIONAL }
RecipientInfos ::= SET SIZE (1..MAX) OF RecipientInfo
EncryptedContentInfo ::= SEQUENCE {
contentType ContentType,
contentEncryptionAlgorithm ContentEncryptionAlgorithmIdentifier,
encryptedContent [0] IMPLICIT EncryptedContent OPTIONAL }
EncryptedContent ::= OCTET STRING
UnprotectedAttributes ::= SET SIZE (1..MAX) OF Attribute
The fields of type EnvelopedData have the following meanings:
version is the syntax version number. The appropriate value
depends on originatorInfo, RecipientInfo, and unprotectedAttrs.
The version MUST be assigned as follows:
IF (originatorInfo is present) AND
((any certificates with a type of other are present) OR
(any crls with a type of other are present))
THEN version is 4
ELSE
IF ((originatorInfo is present) AND
(any version 2 attribute certificates are present)) OR
(any RecipientInfo structures include pwri) OR
(any RecipientInfo structures include ori)
THEN version is 3
ELSE
IF (originatorInfo is absent) AND
(unprotectedAttrs is absent) AND
(all RecipientInfo structures are version 0)
THEN version is 0
ELSE version is 2
Housley Standards Track [Page 19]
RFC 5652 Cryptographic Message Syntax September 2009
originatorInfo optionally provides information about the
originator. It is present only if required by the key management
algorithm. It may contain certificates and CRLs:
certs is a collection of certificates. certs may contain
originator certificates associated with several different key
management algorithms. certs may also contain attribute
certificates associated with the originator. The certificates
contained in certs are intended to be sufficient for all
recipients to build certification paths from a recognized
"root" or "top-level certification authority". However, certs
may contain more certificates than necessary, and there may be
certificates sufficient to make certification paths from two or
more independent top-level certification authorities.
Alternatively, certs may contain fewer certificates than
necessary, if it is expected that recipients have an alternate
means of obtaining necessary certificates (e.g., from a
previous set of certificates).
crls is a collection of CRLs. It is intended that the set
contain information sufficient to determine whether or not the
certificates in the certs field are valid, but such
correspondence is not necessary. There MAY be more CRLs than
necessary, and there MAY also be fewer CRLs than necessary.
recipientInfos is a collection of per-recipient information.
There MUST be at least one element in the collection.
encryptedContentInfo is the encrypted content information.
unprotectedAttrs is a collection of attributes that are not
encrypted. The field is optional. Useful attribute types are
defined in Section 11.
The fields of type EncryptedContentInfo have the following meanings:
contentType indicates the type of content.
contentEncryptionAlgorithm identifies the content-encryption
algorithm, and any associated parameters, used to encrypt the
content. The content-encryption process is described in Section
6.3. The same content-encryption algorithm and content-encryption
key are used for all recipients.
encryptedContent is the result of encrypting the content. The
field is optional, and if the field is not present, its intended
value must be supplied by other means.
Housley Standards Track [Page 20]
RFC 5652 Cryptographic Message Syntax September 2009
The recipientInfos field comes before the encryptedContentInfo field
so that an EnvelopedData value may be processed in a single pass.
6.2. RecipientInfo Type
Per-recipient information is represented in the type RecipientInfo.
RecipientInfo has a different format for each of the supported key
management techniques. Any of the key management techniques can be
used for each recipient of the same encrypted content. In all cases,
the encrypted content-encryption key is transferred to one or more
recipients.
Since all implementations will not support every possible key
management algorithm, all implementations MUST gracefully handle
unimplemented algorithms when they are encountered. For example, if
a recipient receives a content-encryption key encrypted in their RSA
public key using RSA-OAEP (Optimal Asymmetric Encryption Padding) and
the implementation only supports RSA PKCS #1 v1.5, then a graceful
failure must be implemented.
Implementations MUST support key transport, key agreement, and
previously distributed symmetric key-encryption keys, as represented
by ktri, kari, and kekri, respectively. Implementations MAY support
the password-based key management as represented by pwri.
Implementations MAY support any other key management technique as
represented by ori. Since each recipient can employ a different key
management technique and future specifications could define
additional key management techniques, all implementations MUST
gracefully handle unimplemented alternatives within the RecipientInfo
CHOICE, all implementations MUST gracefully handle unimplemented
versions of otherwise supported alternatives within the RecipientInfo
CHOICE, and all implementations MUST gracefully handle unimplemented
or unknown ori alternatives.
RecipientInfo ::= CHOICE {
ktri KeyTransRecipientInfo,
kari [1] KeyAgreeRecipientInfo,
kekri [2] KEKRecipientInfo,
pwri [3] PasswordRecipientinfo,
ori [4] OtherRecipientInfo }
EncryptedKey ::= OCTET STRING
Housley Standards Track [Page 21]
RFC 5652 Cryptographic Message Syntax September 2009
6.2.1. KeyTransRecipientInfo Type
Per-recipient information using key transport is represented in the
type KeyTransRecipientInfo. Each instance of KeyTransRecipientInfo
transfers the content-encryption key to one recipient.
KeyTransRecipientInfo ::= SEQUENCE {
version CMSVersion, -- always set to 0 or 2
rid RecipientIdentifier,
keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
encryptedKey EncryptedKey }
RecipientIdentifier ::= CHOICE {
issuerAndSerialNumber IssuerAndSerialNumber,
subjectKeyIdentifier [0] SubjectKeyIdentifier }
The fields of type KeyTransRecipientInfo have the following meanings:
version is the syntax version number. If the RecipientIdentifier
is the CHOICE issuerAndSerialNumber, then the version MUST be 0.
If the RecipientIdentifier is subjectKeyIdentifier, then the
version MUST be 2.
rid specifies the recipient's certificate or key that was used by
the sender to protect the content-encryption key. The content-
encryption key is encrypted with the recipient's public key. The
RecipientIdentifier provides two alternatives for specifying the
recipient's certificate, and thereby the recipient's public key.
The recipient's certificate must contain a key transport public
key. Therefore, a recipient X.509 version 3 certificate that
contains a key usage extension MUST assert the keyEncipherment
bit. The issuerAndSerialNumber alternative identifies the
recipient's certificate by the issuer's distinguished name and the
certificate serial number; the subjectKeyIdentifier identifies the
recipient's certificate by a key identifier. When an X.509
certificate is referenced, the key identifier matches the X.509
subjectKeyIdentifier extension value. When other certificate
formats are referenced, the documents that specify the certificate
format and their use with the CMS must include details on matching
the key identifier to the appropriate certificate field. For
recipient processing, implementations MUST support both of these
alternatives for specifying the recipient's certificate. For
sender processing, implementations MUST support at least one of
these alternatives.
Housley Standards Track [Page 22]
RFC 5652 Cryptographic Message Syntax September 2009
keyEncryptionAlgorithm identifies the key-encryption algorithm,
and any associated parameters, used to encrypt the content-
encryption key for the recipient. The key-encryption process is
described in Section 6.4.
encryptedKey is the result of encrypting the content-encryption
key for the recipient.
6.2.2. KeyAgreeRecipientInfo Type
Recipient information using key agreement is represented in the type
KeyAgreeRecipientInfo. Each instance of KeyAgreeRecipientInfo will
transfer the content-encryption key to one or more recipients that
use the same key agreement algorithm and domain parameters for that
algorithm.
KeyAgreeRecipientInfo ::= SEQUENCE {
version CMSVersion, -- always set to 3
originator [0] EXPLICIT OriginatorIdentifierOrKey,
ukm [1] EXPLICIT UserKeyingMaterial OPTIONAL,
keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
recipientEncryptedKeys RecipientEncryptedKeys }
OriginatorIdentifierOrKey ::= CHOICE {
issuerAndSerialNumber IssuerAndSerialNumber,
subjectKeyIdentifier [0] SubjectKeyIdentifier,
originatorKey [1] OriginatorPublicKey }
OriginatorPublicKey ::= SEQUENCE {
algorithm AlgorithmIdentifier,
publicKey BIT STRING }
RecipientEncryptedKeys ::= SEQUENCE OF RecipientEncryptedKey
RecipientEncryptedKey ::= SEQUENCE {
rid KeyAgreeRecipientIdentifier,
encryptedKey EncryptedKey }
KeyAgreeRecipientIdentifier ::= CHOICE {
issuerAndSerialNumber IssuerAndSerialNumber,
rKeyId [0] IMPLICIT RecipientKeyIdentifier }
RecipientKeyIdentifier ::= SEQUENCE {
subjectKeyIdentifier SubjectKeyIdentifier,
date GeneralizedTime OPTIONAL,
other OtherKeyAttribute OPTIONAL }
SubjectKeyIdentifier ::= OCTET STRING
Housley Standards Track [Page 23]
RFC 5652 Cryptographic Message Syntax September 2009
The fields of type KeyAgreeRecipientInfo have the following meanings:
version is the syntax version number. It MUST always be 3.
originator is a CHOICE with three alternatives specifying the
sender's key agreement public key. The sender uses the
corresponding private key and the recipient's public key to
generate a pairwise key. The content-encryption key is encrypted
in the pairwise key. The issuerAndSerialNumber alternative
identifies the sender's certificate, and thereby the sender's
public key, by the issuer's distinguished name and the certificate
serial number. The subjectKeyIdentifier alternative identifies
the sender's certificate, and thereby the sender's public key, by
a key identifier. When an X.509 certificate is referenced, the
key identifier matches the X.509 subjectKeyIdentifier extension
value. When other certificate formats are referenced, the
documents that specify the certificate format and their use with
the CMS must include details on matching the key identifier to the
appropriate certificate field. The originatorKey alternative
includes the algorithm identifier and sender's key agreement
public key. This alternative permits originator anonymity since
the public key is not certified. Implementations MUST support all
three alternatives for specifying the sender's public key.
ukm is optional. With some key agreement algorithms, the sender
provides a User Keying Material (UKM) to ensure that a different
key is generated each time the same two parties generate a
pairwise key. Implementations MUST accept a KeyAgreeRecipientInfo
SEQUENCE that includes a ukm field. Implementations that do not
support key agreement algorithms that make use of UKMs MUST
gracefully handle the presence of UKMs.
keyEncryptionAlgorithm identifies the key-encryption algorithm,
and any associated parameters, used to encrypt the content-
encryption key with the key-encryption key. The key-encryption
process is described in Section 6.4.
recipientEncryptedKeys includes a recipient identifier and
encrypted key for one or more recipients. The
KeyAgreeRecipientIdentifier is a CHOICE with two alternatives
specifying the recipient's certificate, and thereby the
recipient's public key, that was used by the sender to generate a
pairwise key-encryption key. The recipient's certificate must
contain a key agreement public key. Therefore, a recipient X.509
version 3 certificate that contains a key usage extension MUST
assert the keyAgreement bit. The content-encryption key is
encrypted in the pairwise key-encryption key. The
issuerAndSerialNumber alternative identifies the recipient's
Housley Standards Track [Page 24]
RFC 5652 Cryptographic Message Syntax September 2009
certificate by the issuer's distinguished name and the certificate
serial number; the RecipientKeyIdentifier is described below. The
encryptedKey is the result of encrypting the content-encryption
key in the pairwise key-encryption key generated using the key
agreement algorithm. Implementations MUST support both
alternatives for specifying the recipient's certificate.
The fields of type RecipientKeyIdentifier have the following
meanings:
subjectKeyIdentifier identifies the recipient's certificate by a
key identifier. When an X.509 certificate is referenced, the key
identifier matches the X.509 subjectKeyIdentifier extension value.
When other certificate formats are referenced, the documents that
specify the certificate format and their use with the CMS must
include details on matching the key identifier to the appropriate
certificate field.
date is optional. When present, the date specifies which of the
recipient's previously distributed UKMs was used by the sender.
other is optional. When present, this field contains additional
information used by the recipient to locate the public keying
material used by the sender.
6.2.3. KEKRecipientInfo Type
Recipient information using previously distributed symmetric keys is
represented in the type KEKRecipientInfo. Each instance of
KEKRecipientInfo will transfer the content-encryption key to one or
more recipients who have the previously distributed key-encryption
key.
KEKRecipientInfo ::= SEQUENCE {
version CMSVersion, -- always set to 4
kekid KEKIdentifier,
keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
encryptedKey EncryptedKey }
KEKIdentifier ::= SEQUENCE {
keyIdentifier OCTET STRING,
date GeneralizedTime OPTIONAL,
other OtherKeyAttribute OPTIONAL }
Housley Standards Track [Page 25]
RFC 5652 Cryptographic Message Syntax September 2009
The fields of type KEKRecipientInfo have the following meanings:
version is the syntax version number. It MUST always be 4.
kekid specifies a symmetric key-encryption key that was previously
distributed to the sender and one or more recipients.
keyEncryptionAlgorithm identifies the key-encryption algorithm,
and any associated parameters, used to encrypt the content-
encryption key with the key-encryption key. The key-encryption
process is described in Section 6.4.
encryptedKey is the result of encrypting the content-encryption
key in the key-encryption key.
The fields of type KEKIdentifier have the following meanings:
keyIdentifier identifies the key-encryption key that was
previously distributed to the sender and one or more recipients.
date is optional. When present, the date specifies a single key-
encryption key from a set that was previously distributed.
other is optional. When present, this field contains additional
information used by the recipient to determine the key-encryption
key used by the sender.
6.2.4. PasswordRecipientInfo Type
Recipient information using a password or shared secret value is
represented in the type PasswordRecipientInfo. Each instance of
PasswordRecipientInfo will transfer the content-encryption key to one
or more recipients who possess the password or shared secret value.
The PasswordRecipientInfo Type is specified in RFC 3211 [PWRI]. The
PasswordRecipientInfo structure is repeated here for completeness.
PasswordRecipientInfo ::= SEQUENCE {
version CMSVersion, -- Always set to 0
keyDerivationAlgorithm [0] KeyDerivationAlgorithmIdentifier
OPTIONAL,
keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
encryptedKey EncryptedKey }
Housley Standards Track [Page 26]
RFC 5652 Cryptographic Message Syntax September 2009
The fields of type PasswordRecipientInfo have the following meanings:
version is the syntax version number. It MUST always be 0.
keyDerivationAlgorithm identifies the key-derivation algorithm,
and any associated parameters, used to derive the key-encryption
key from the password or shared secret value. If this field is
absent, the key-encryption key is supplied from an external
source, for example a hardware crypto token such as a smart card.
keyEncryptionAlgorithm identifies the encryption algorithm, and
any associated parameters, used to encrypt the content-encryption
key with the key-encryption key.
encryptedKey is the result of encrypting the content-encryption
key with the key-encryption key.
6.2.5. OtherRecipientInfo Type
Recipient information for additional key management techniques are
represented in the type OtherRecipientInfo. The OtherRecipientInfo
type allows key management techniques beyond key transport, key
agreement, previously distributed symmetric key-encryption keys, and
password-based key management to be specified in future documents.
An object identifier uniquely identifies such key management
techniques.
OtherRecipientInfo ::= SEQUENCE {
oriType OBJECT IDENTIFIER,
oriValue ANY DEFINED BY oriType }
The fields of type OtherRecipientInfo have the following meanings:
oriType identifies the key management technique.
oriValue contains the protocol data elements needed by a recipient
using the identified key management technique.
6.3. Content-encryption Process
The content-encryption key for the desired content-encryption
algorithm is randomly generated. The data to be protected is padded
as described below, then the padded data is encrypted using the
content-encryption key. The encryption operation maps an arbitrary
string of octets (the data) to another string of octets (the
ciphertext) under control of a content-encryption key. The encrypted
data is included in the EnvelopedData encryptedContentInfo
encryptedContent OCTET STRING.
Housley Standards Track [Page 27]
RFC 5652 Cryptographic Message Syntax September 2009
Some content-encryption algorithms assume the input length is a
multiple of k octets, where k is greater than one. For such
algorithms, the input shall be padded at the trailing end with
k-(lth mod k) octets all having value k-(lth mod k), where lth is
the length of the input. In other words, the input is padded at
the trailing end with one of the following strings:
01 -- if lth mod k = k-1
02 02 -- if lth mod k = k-2
.
.
.
k k ... k k -- if lth mod k = 0
The padding can be removed unambiguously since all input is padded,
including input values that are already a multiple of the block size,
and no padding string is a suffix of another. This padding method is
well defined if and only if k is less than 256.
6.4. Key-encryption Process
The input to the key-encryption process -- the value supplied to the
recipient's key-encryption algorithm -- is just the "value" of the
content-encryption key.
Any of the aforementioned key management techniques can be used for
each recipient of the same encrypted content.
7. Digested-data Content Type
The digested-data content type consists of content of any type and a
message digest of the content.
Typically, the digested-data content type is used to provide content
integrity, and the result generally becomes an input to the
enveloped-data content type.
The following steps construct digested-data:
1. A message digest is computed on the content with a message-digest
algorithm.
2. The message-digest algorithm and the message digest are collected
together with the content into a DigestedData value.
A recipient verifies the message digest by comparing the message
digest to an independently computed message digest.
Housley Standards Track [Page 28]
RFC 5652 Cryptographic Message Syntax September 2009
The following object identifier identifies the digested-data content
type:
id-digestedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 5 }
The digested-data content type shall have ASN.1 type DigestedData:
DigestedData ::= SEQUENCE {
version CMSVersion,
digestAlgorithm DigestAlgorithmIdentifier,
encapContentInfo EncapsulatedContentInfo,
digest Digest }
Digest ::= OCTET STRING
The fields of type DigestedData have the following meanings:
version is the syntax version number. If the encapsulated content
type is id-data, then the value of version MUST be 0; however, if
the encapsulated content type is other than id-data, then the
value of version MUST be 2.
digestAlgorithm identifies the message digest algorithm, and any
associated parameters, under which the content is digested. The
message-digesting process is the same as in Section 5.4 in the
case when there are no signed attributes.
encapContentInfo is the content that is digested, as defined in
Section 5.2.
digest is the result of the message-digesting process.
The ordering of the digestAlgorithm field, the encapContentInfo
field, and the digest field makes it possible to process a
DigestedData value in a single pass.
8. Encrypted-data Content Type
The encrypted-data content type consists of encrypted content of any
type. Unlike the enveloped-data content type, the encrypted-data
content type has neither recipients nor encrypted content-encryption
keys. Keys MUST be managed by other means.
The typical application of the encrypted-data content type will be to
encrypt the content of the data content type for local storage,
perhaps where the encryption key is derived from a password.
Housley Standards Track [Page 29]
RFC 5652 Cryptographic Message Syntax September 2009
The following object identifier identifies the encrypted-data content
type:
id-encryptedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 6 }
The encrypted-data content type shall have ASN.1 type EncryptedData:
EncryptedData ::= SEQUENCE {
version CMSVersion,
encryptedContentInfo EncryptedContentInfo,
unprotectedAttrs [1] IMPLICIT UnprotectedAttributes OPTIONAL }
The fields of type EncryptedData have the following meanings:
version is the syntax version number. If unprotectedAttrs is
present, then the version MUST be 2. If unprotectedAttrs is
absent, then version MUST be 0.
encryptedContentInfo is the encrypted content information, as
defined in Section 6.1.
unprotectedAttrs is a collection of attributes that are not
encrypted. The field is optional. Useful attribute types are
defined in Section 11.
9. Authenticated-data Content Type
The authenticated-data content type consists of content of any type,
a message authentication code (MAC), and encrypted authentication
keys for one or more recipients. The combination of the MAC and one
encrypted authentication key for a recipient is necessary for that
recipient to verify the integrity of the content. Any type of
content can be integrity protected for an arbitrary number of
recipients.
The process by which authenticated-data is constructed involves the
following steps:
1. A message-authentication key for a particular message-
authentication algorithm is generated at random.
2. The message-authentication key is encrypted for each recipient.
The details of this encryption depend on the key management
algorithm used.
Housley Standards Track [Page 30]
RFC 5652 Cryptographic Message Syntax September 2009
3. For each recipient, the encrypted message-authentication key and
other recipient-specific information are collected into a
RecipientInfo value, defined in Section 6.2.
4. Using the message-authentication key, the originator computes a
MAC value on the content. If the originator is authenticating
any information in addition to the content (see Section 9.2), a
message digest is calculated on the content, the message digest
of the content and the other information are authenticated using
the message-authentication key, and the result becomes the "MAC
value".
9.1. AuthenticatedData Type
The following object identifier identifies the authenticated-data
content type:
id-ct-authData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16)
ct(1) 2 }
The authenticated-data content type shall have ASN.1 type
AuthenticatedData:
AuthenticatedData ::= SEQUENCE {
version CMSVersion,
originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL,
recipientInfos RecipientInfos,
macAlgorithm MessageAuthenticationCodeAlgorithm,
digestAlgorithm [1] DigestAlgorithmIdentifier OPTIONAL,
encapContentInfo EncapsulatedContentInfo,
authAttrs [2] IMPLICIT AuthAttributes OPTIONAL,
mac MessageAuthenticationCode,
unauthAttrs [3] IMPLICIT UnauthAttributes OPTIONAL }
AuthAttributes ::= SET SIZE (1..MAX) OF Attribute
UnauthAttributes ::= SET SIZE (1..MAX) OF Attribute
MessageAuthenticationCode ::= OCTET STRING
The fields of type AuthenticatedData have the following meanings:
version is the syntax version number. The version MUST be
assigned as follows:
Housley Standards Track [Page 31]
RFC 5652 Cryptographic Message Syntax September 2009
IF (originatorInfo is present) AND
((any certificates with a type of other are present) OR
(any crls with a type of other are present))
THEN version is 3
ELSE
IF ((originatorInfo is present) AND
(any version 2 attribute certificates are present))
THEN version is 1
ELSE version is 0
originatorInfo optionally provides information about the
originator. It is present only if required by the key management
algorithm. It MAY contain certificates, attribute certificates,
and CRLs, as defined in Section 6.1.
recipientInfos is a collection of per-recipient information, as
defined in Section 6.1. There MUST be at least one element in the
collection.
macAlgorithm is a message authentication code (MAC) algorithm
identifier. It identifies the MAC algorithm, along with any
associated parameters, used by the originator. Placement of the
macAlgorithm field facilitates one-pass processing by the
recipient.
digestAlgorithm identifies the message digest algorithm, and any
associated parameters, used to compute a message digest on the
encapsulated content if authenticated attributes are present. The
message digesting process is described in Section 9.2. Placement
of the digestAlgorithm field facilitates one-pass processing by
the recipient. If the digestAlgorithm field is present, then the
authAttrs field MUST also be present.
encapContentInfo is the content that is authenticated, as defined
in Section 5.2.
authAttrs is a collection of authenticated attributes. The
authAttrs structure is optional, but it MUST be present if the
content type of the EncapsulatedContentInfo value being
authenticated is not id-data. If the authAttrs field is present,
then the digestAlgorithm field MUST also be present. The
AuthAttributes structure MUST be DER encoded, even if the rest of
the structure is BER encoded. Useful attribute types are defined
in Section 11. If the authAttrs field is present, it MUST
contain, at a minimum, the following two attributes:
Housley Standards Track [Page 32]
RFC 5652 Cryptographic Message Syntax September 2009
A content-type attribute having as its value the content type
of the EncapsulatedContentInfo value being authenticated.
Section 11.1 defines the content-type attribute.
A message-digest attribute, having as its value the message
digest of the content. Section 11.2 defines the message-digest
attribute.
mac is the message authentication code.
unauthAttrs is a collection of attributes that are not
authenticated. The field is optional. To date, no attributes
have been defined for use as unauthenticated attributes, but other
useful attribute types are defined in Section 11.
9.2. MAC Generation
The MAC calculation process computes a message authentication code
(MAC) on either the content being authenticated or a message digest
of content being authenticated together with the originator's
authenticated attributes.
If the authAttrs field is absent, the input to the MAC calculation
process is the value of the encapContentInfo eContent OCTET STRING.
Only the octets comprising the value of the eContent OCTET STRING are
input to the MAC algorithm; the tag and the length octets are
omitted. This has the advantage that the length of the content being
authenticated need not be known in advance of the MAC generation
process.
If the authAttrs field is present, the content-type attribute (as
described in Section 11.1) and the message-digest attribute (as
described in Section 11.2) MUST be included, and the input to the MAC
calculation process is the DER encoding of authAttrs. A separate
encoding of the authAttrs field is performed for message digest
calculation. The IMPLICIT [2] tag in the authAttrs field is not used
for the DER encoding, rather an EXPLICIT SET OF tag is used. That
is, the DER encoding of the SET OF tag, rather than of the IMPLICIT
[2] tag, is to be included in the message digest calculation along
with the length and content octets of the authAttrs value.
The message digest calculation process computes a message digest on
the content being authenticated. The initial input to the message
digest calculation process is the "value" of the encapsulated content
being authenticated. Specifically, the input is the encapContentInfo
eContent OCTET STRING to which the authentication process is applied.
Only the octets comprising the value of the encapContentInfo eContent
OCTET STRING are input to the message digest algorithm, not the tag
Housley Standards Track [Page 33]
RFC 5652 Cryptographic Message Syntax September 2009
or the length octets. This has the advantage that the length of the
content being authenticated need not be known in advance. Although
the encapContentInfo eContent OCTET STRING tag and length octets are
not included in the message digest calculation, they are still
protected by other means. The length octets are protected by the
nature of the message digest algorithm since it is computationally
infeasible to find any two distinct contents of any length that have
the same message digest.
The input to the MAC calculation process includes the MAC input data,
defined above, and an authentication key conveyed in a recipientInfo
structure. The details of MAC calculation depend on the MAC
algorithm employed (e.g., Hashed Message Authentication Code (HMAC)).
The object identifier, along with any parameters, that specifies the
MAC algorithm employed by the originator is carried in the
macAlgorithm field. The MAC value generated by the originator is
encoded as an OCTET STRING and carried in the mac field.
9.3. MAC Verification
The input to the MAC verification process includes the input data
(determined based on the presence or absence of the authAttrs field,
as defined in 9.2), and the authentication key conveyed in
recipientInfo. The details of the MAC verification process depend on
the MAC algorithm employed.
The recipient MUST NOT rely on any MAC values or message digest
values computed by the originator. The content is authenticated as
described in Section 9.2. If the originator includes authenticated
attributes, then the content of the authAttrs is authenticated as
described in Section 9.2. For authentication to succeed, the MAC
value calculated by the recipient MUST be the same as the value of
the mac field. Similarly, for authentication to succeed when the
authAttrs field is present, the content message digest value
calculated by the recipient MUST be the same as the message digest
value included in the authAttrs message-digest attribute.
If the AuthenticatedData includes authAttrs, then the content-type
attribute value MUST match the AuthenticatedData encapContentInfo
eContentType value.
10. Useful Types
This section is divided into two parts. The first part defines
algorithm identifiers, and the second part defines other useful
types.
Housley Standards Track [Page 34]
RFC 5652 Cryptographic Message Syntax September 2009
10.1. Algorithm Identifier Types
All of the algorithm identifiers have the same type:
AlgorithmIdentifier. The definition of AlgorithmIdentifier is taken
from X.509 [X.509-88].
There are many alternatives for each algorithm type.
10.1.1. DigestAlgorithmIdentifier
The DigestAlgorithmIdentifier type identifies a message-digest
algorithm. Examples include SHA-1, MD2, and MD5. A message-digest
algorithm maps an octet string (the content) to another octet string
(the message digest).
DigestAlgorithmIdentifier ::= AlgorithmIdentifier
10.1.2. SignatureAlgorithmIdentifier
The SignatureAlgorithmIdentifier type identifies a signature
algorithm, and it can also identify a message digest algorithm.
Examples include RSA, DSA, DSA with SHA-1, ECDSA, and ECDSA with
SHA-256. A signature algorithm supports signature generation and
verification operations. The signature generation operation uses the
message digest and the signer's private key to generate a signature
value. The signature verification operation uses the message digest
and the signer's public key to determine whether or not a signature
value is valid. Context determines which operation is intended.
SignatureAlgorithmIdentifier ::= AlgorithmIdentifier
10.1.3. KeyEncryptionAlgorithmIdentifier
The KeyEncryptionAlgorithmIdentifier type identifies a key-encryption
algorithm used to encrypt a content-encryption key. The encryption
operation maps an octet string (the key) to another octet string (the
encrypted key) under control of a key-encryption key. The decryption
operation is the inverse of the encryption operation. Context
determines which operation is intended.
The details of encryption and decryption depend on the key management
algorithm used. Key transport, key agreement, previously distributed
symmetric key-encrypting keys, and symmetric key-encrypting keys
derived from passwords are supported.
KeyEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier
Housley Standards Track [Page 35]
RFC 5652 Cryptographic Message Syntax September 2009
10.1.4. ContentEncryptionAlgorithmIdentifier
The ContentEncryptionAlgorithmIdentifier type identifies a content-
encryption algorithm. Examples include Triple-DES and RC2. A
content-encryption algorithm supports encryption and decryption
operations. The encryption operation maps an octet string (the
plaintext) to another octet string (the ciphertext) under control of
a content-encryption key. The decryption operation is the inverse of
the encryption operation. Context determines which operation is
intended.
ContentEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier
10.1.5. MessageAuthenticationCodeAlgorithm
The MessageAuthenticationCodeAlgorithm type identifies a message
authentication code (MAC) algorithm. Examples include DES-MAC and
HMAC-SHA-1. A MAC algorithm supports generation and verification
operations. The MAC generation and verification operations use the
same symmetric key. Context determines which operation is intended.
MessageAuthenticationCodeAlgorithm ::= AlgorithmIdentifier
10.1.6. KeyDerivationAlgorithmIdentifier
The KeyDerivationAlgorithmIdentifier type is specified in RFC 3211
[PWRI]. The KeyDerivationAlgorithmIdentifier definition is repeated
here for completeness.
Key derivation algorithms convert a password or shared secret value
into a key-encryption key.
KeyDerivationAlgorithmIdentifier ::= AlgorithmIdentifier
10.2. Other Useful Types
This section defines types that are used other places in the
document. The types are not listed in any particular order.
10.2.1. RevocationInfoChoices
The RevocationInfoChoices type gives a set of revocation status
information alternatives. It is intended that the set contain
information sufficient to determine whether the certificates and
attribute certificates with which the set is associated are revoked.
However, there MAY be more revocation status information than
necessary or there MAY be less revocation status information than
necessary. X.509 Certificate revocation lists (CRLs) [X.509-97] are
Housley Standards Track [Page 36]
RFC 5652 Cryptographic Message Syntax September 2009
the primary source of revocation status information, but any other
revocation information format can be supported. The
OtherRevocationInfoFormat alternative is provided to support any
other revocation information format without further modifications to
the CMS. For example, Online Certificate Status Protocol (OCSP)
Responses [OCSP] can be supported using the
OtherRevocationInfoFormat.
The CertificateList may contain a CRL, an Authority Revocation List
(ARL), a Delta CRL, or an Attribute Certificate Revocation List. All
of these lists share a common syntax.
The CertificateList type gives a certificate revocation list (CRL).
CRLs are specified in X.509 [X.509-97], and they are profiled for use
in the Internet in RFC 5280 [PROFILE].
The definition of CertificateList is taken from X.509.
RevocationInfoChoices ::= SET OF RevocationInfoChoice
RevocationInfoChoice ::= CHOICE {
crl CertificateList,
other [1] IMPLICIT OtherRevocationInfoFormat }
OtherRevocationInfoFormat ::= SEQUENCE {
otherRevInfoFormat OBJECT IDENTIFIER,
otherRevInfo ANY DEFINED BY otherRevInfoFormat }
10.2.2. CertificateChoices
The CertificateChoices type gives either a PKCS #6 extended
certificate [PKCS#6], an X.509 certificate, a version 1 X.509
attribute certificate (ACv1) [X.509-97], a version 2 X.509 attribute
certificate (ACv2) [X.509-00], or any other certificate format. The
PKCS #6 extended certificate is obsolete. The PKCS #6 certificate is
included for backward compatibility, and PKCS #6 certificates SHOULD
NOT be used. The ACv1 is also obsolete. ACv1 is included for
backward compatibility, and ACv1 SHOULD NOT be used. The Internet
profile of X.509 certificates is specified in the "Internet X.509
Public Key Infrastructure: Certificate and CRL Profile" [PROFILE].
The Internet profile of ACv2 is specified in the "An Internet
Attribute Certificate Profile for Authorization" [ACPROFILE]. The
OtherCertificateFormat alternative is provided to support any other
certificate format without further modifications to the CMS.
The definition of Certificate is taken from X.509.
Housley Standards Track [Page 37]
RFC 5652 Cryptographic Message Syntax September 2009
The definitions of AttributeCertificate are taken from X.509-1997 and
X.509-2000. The definition from X.509-1997 is assigned to
AttributeCertificateV1 (see Section 12.2), and the definition from
X.509-2000 is assigned to AttributeCertificateV2.
CertificateChoices ::= CHOICE {
certificate Certificate,
extendedCertificate [0] IMPLICIT ExtendedCertificate, -- Obsolete
v1AttrCert [1] IMPLICIT AttributeCertificateV1, -- Obsolete
v2AttrCert [2] IMPLICIT AttributeCertificateV2,
other [3] IMPLICIT OtherCertificateFormat }
OtherCertificateFormat ::= SEQUENCE {
otherCertFormat OBJECT IDENTIFIER,
otherCert ANY DEFINED BY otherCertFormat }
10.2.3. CertificateSet
The CertificateSet type provides a set of certificates. It is
intended that the set be sufficient to contain certification paths
from a recognized "root" or "top-level certification authority" to
all of the sender certificates with which the set is associated.
However, there may be more certificates than necessary, or there MAY
be fewer than necessary.
The precise meaning of a "certification path" is outside the scope of
this document. However, [PROFILE] provides a definition for X.509
certificates. Some applications may impose upper limits on the
length of a certification path; others may enforce certain
relationships between the subjects and issuers of certificates within
a certification path.
CertificateSet ::= SET OF CertificateChoices
10.2.4. IssuerAndSerialNumber
The IssuerAndSerialNumber type identifies a certificate, and thereby
an entity and a public key, by the distinguished name of the
certificate issuer and an issuer-specific certificate serial number.
The definition of Name is taken from X.501 [X.501-88], and the
definition of CertificateSerialNumber is taken from X.509 [X.509-97].
IssuerAndSerialNumber ::= SEQUENCE {
issuer Name,
serialNumber CertificateSerialNumber }
CertificateSerialNumber ::= INTEGER
Housley Standards Track [Page 38]
RFC 5652 Cryptographic Message Syntax September 2009
10.2.5. CMSVersion
The CMSVersion type gives a syntax version number, for compatibility
with future revisions of this specification.
CMSVersion ::= INTEGER
{ v0(0), v1(1), v2(2), v3(3), v4(4), v5(5) }
10.2.6. UserKeyingMaterial
The UserKeyingMaterial type gives a syntax for user keying material
(UKM). Some key agreement algorithms require UKMs to ensure that a
different key is generated each time the same two parties generate a
pairwise key. The sender provides a UKM for use with a specific key
agreement algorithm.
UserKeyingMaterial ::= OCTET STRING
10.2.7. OtherKeyAttribute
The OtherKeyAttribute type gives a syntax for the inclusion of other
key attributes that permit the recipient to select the key used by
the sender. The attribute object identifier must be registered along
with the syntax of the attribute itself. Use of this structure
should be avoided since it might impede interoperability.
OtherKeyAttribute ::= SEQUENCE {
keyAttrId OBJECT IDENTIFIER,
keyAttr ANY DEFINED BY keyAttrId OPTIONAL }
11. Useful Attributes
This section defines attributes that may be used with signed-data,
enveloped-data, encrypted-data, or authenticated-data. The syntax of
Attribute is compatible with X.501 [X.501-88] and RFC 5280 [PROFILE].
Some of the attributes defined in this section were originally
defined in PKCS #9 [PKCS#9]; others were originally defined in a
previous version of this specification [CMS1]. The attributes are
not listed in any particular order.
Additional attributes are defined in many places, notably the S/MIME
Version 3.1 Message Specification [MSG3.1] and the Enhanced Security
Services for S/MIME [ESS], which also include recommendations on the
placement of these attributes.
Housley Standards Track [Page 39]
RFC 5652 Cryptographic Message Syntax September 2009
11.1. Content Type
The content-type attribute type specifies the content type of the
ContentInfo within signed-data or authenticated-data. The content-
type attribute type MUST be present whenever signed attributes are
present in signed-data or authenticated attributes present in
authenticated-data. The content-type attribute value MUST match the
encapContentInfo eContentType value in the signed-data or
authenticated-data.
The content-type attribute MUST be a signed attribute or an
authenticated attribute; it MUST NOT be an unsigned attribute,
unauthenticated attribute, or unprotected attribute.
The following object identifier identifies the content-type
attribute:
id-contentType OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) 3 }
Content-type attribute values have ASN.1 type ContentType:
ContentType ::= OBJECT IDENTIFIER
Even though the syntax is defined as a SET OF AttributeValue, a
content-type attribute MUST have a single attribute value; zero or
multiple instances of AttributeValue are not permitted.
The SignedAttributes and AuthAttributes syntaxes are each defined as
a SET OF Attributes. The SignedAttributes in a signerInfo MUST NOT
include multiple instances of the content-type attribute. Similarly,
the AuthAttributes in an AuthenticatedData MUST NOT include multiple
instances of the content-type attribute.
11.2. Message Digest
The message-digest attribute type specifies the message digest of the
encapContentInfo eContent OCTET STRING being signed in signed-data
(see Section 5.4) or authenticated in authenticated-data (see Section
9.2). For signed-data, the message digest is computed using the
signer's message digest algorithm. For authenticated-data, the
message digest is computed using the originator's message digest
algorithm.
Within signed-data, the message-digest signed attribute type MUST be
present when there are any signed attributes present. Within
authenticated-data, the message-digest authenticated attribute type
MUST be present when there are any authenticated attributes present.
Housley Standards Track [Page 40]
RFC 5652 Cryptographic Message Syntax September 2009
The message-digest attribute MUST be a signed attribute or an
authenticated attribute; it MUST NOT be an unsigned attribute,
unauthenticated attribute, or unprotected attribute.
The following object identifier identifies the message-digest
attribute:
id-messageDigest OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) 4 }
Message-digest attribute values have ASN.1 type MessageDigest:
MessageDigest ::= OCTET STRING
A message-digest attribute MUST have a single attribute value, even
though the syntax is defined as a SET OF AttributeValue. There MUST
NOT be zero or multiple instances of AttributeValue present.
The SignedAttributes syntax and AuthAttributes syntax are each
defined as a SET OF Attributes. The SignedAttributes in a signerInfo
MUST include only one instance of the message-digest attribute.
Similarly, the AuthAttributes in an AuthenticatedData MUST include
only one instance of the message-digest attribute.
11.3. Signing Time
The signing-time attribute type specifies the time at which the
signer (purportedly) performed the signing process. The signing-time
attribute type is intended for use in signed-data.
The signing-time attribute MUST be a signed attribute or an
authenticated attribute; it MUST NOT be an unsigned attribute,
unauthenticated attribute, or unprotected attribute.
The following object identifier identifies the signing-time
attribute:
id-signingTime OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) 5 }
Signing-time attribute values have ASN.1 type SigningTime:
SigningTime ::= Time
Time ::= CHOICE {
utcTime UTCTime,
generalizedTime GeneralizedTime }
Housley Standards Track [Page 41]
RFC 5652 Cryptographic Message Syntax September 2009
Note: The definition of Time matches the one specified in the 1997
version of X.509 [X.509-97].
Dates between 1 January 1950 and 31 December 2049 (inclusive) MUST be
encoded as UTCTime. Any dates with year values before 1950 or after
2049 MUST be encoded as GeneralizedTime.
UTCTime values MUST be expressed in Coordinated Universal Time
(formerly known as Greenwich Mean Time (GMT) and Zulu clock time) and
MUST include seconds (i.e., times are YYMMDDHHMMSSZ), even where the
number of seconds is zero. Midnight MUST be represented as
"YYMMDD000000Z". Century information is implicit, and the century
MUST be determined as follows:
Where YY is greater than or equal to 50, the year MUST be
interpreted as 19YY; and
Where YY is less than 50, the year MUST be interpreted as 20YY.
GeneralizedTime values MUST be expressed in Coordinated Universal
Time and MUST include seconds (i.e., times are YYYYMMDDHHMMSSZ), even
where the number of seconds is zero. GeneralizedTime values MUST NOT
include fractional seconds.
A signing-time attribute MUST have a single attribute value, even
though the syntax is defined as a SET OF AttributeValue. There MUST
NOT be zero or multiple instances of AttributeValue present.
The SignedAttributes syntax and the AuthAttributes syntax are each
defined as a SET OF Attributes. The SignedAttributes in a signerInfo
MUST NOT include multiple instances of the signing-time attribute.
Similarly, the AuthAttributes in an AuthenticatedData MUST NOT
include multiple instances of the signing-time attribute.
No requirement is imposed concerning the correctness of the signing
time, and acceptance of a purported signing time is a matter of a
recipient's discretion. It is expected, however, that some signers,
such as time-stamp servers, will be trusted implicitly.
11.4. Countersignature
The countersignature attribute type specifies one or more signatures
on the contents octets of the signature OCTET STRING in a SignerInfo
value of the signed-data. That is, the message digest is computed
over the octets comprising the value of the OCTET STRING, neither the
tag nor length octets are included. Thus, the countersignature
attribute type countersigns (signs in serial) another signature.
Housley Standards Track [Page 42]
RFC 5652 Cryptographic Message Syntax September 2009
The countersignature attribute MUST be an unsigned attribute; it MUST
NOT be a signed attribute, an authenticated attribute, an
unauthenticated attribute, or an unprotected attribute.
The following object identifier identifies the countersignature
attribute:
id-countersignature OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) 6 }
Countersignature attribute values have ASN.1 type Countersignature:
Countersignature ::= SignerInfo
Countersignature values have the same meaning as SignerInfo values
for ordinary signatures, except that:
1. The signedAttributes field MUST NOT contain a content-type
attribute; there is no content type for countersignatures.
2. The signedAttributes field MUST contain a message-digest
attribute if it contains any other attributes.
3. The input to the message-digesting process is the contents octets
of the DER encoding of the signatureValue field of the SignerInfo
value with which the attribute is associated.
A countersignature attribute can have multiple attribute values. The
syntax is defined as a SET OF AttributeValue, and there MUST be one
or more instances of AttributeValue present.
The UnsignedAttributes syntax is defined as a SET OF Attributes. The
UnsignedAttributes in a signerInfo may include multiple instances of
the countersignature attribute.
A countersignature, since it has type SignerInfo, can itself contain
a countersignature attribute. Thus, it is possible to construct an
arbitrarily long series of countersignatures.
12. ASN.1 Modules
Section 12.1 contains the ASN.1 module for the CMS, and Section 12.2
contains the ASN.1 module for the Version 1 Attribute Certificate.
Housley Standards Track [Page 43]
RFC 5652 Cryptographic Message Syntax September 2009
12.1. CMS ASN.1 Module
CryptographicMessageSyntax2004
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) cms-2004(24) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
-- EXPORTS All
-- The types and values defined in this module are exported for use
-- in the other ASN.1 modules. Other applications may use them for
-- their own purposes.
IMPORTS
-- Imports from RFC 5280 [PROFILE], Appendix A.1
AlgorithmIdentifier, Certificate, CertificateList,
CertificateSerialNumber, Name
FROM PKIX1Explicit88
{ iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7)
mod(0) pkix1-explicit(18) }
-- Imports from RFC 3281 [ACPROFILE], Appendix B
AttributeCertificate
FROM PKIXAttributeCertificate
{ iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7)
mod(0) attribute-cert(12) }
-- Imports from Appendix B of this document
AttributeCertificateV1
FROM AttributeCertificateVersion1
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0)
v1AttrCert(15) } ;
-- Cryptographic Message Syntax
ContentInfo ::= SEQUENCE {
contentType ContentType,
content [0] EXPLICIT ANY DEFINED BY contentType }
ContentType ::= OBJECT IDENTIFIER
Housley Standards Track [Page 44]
RFC 5652 Cryptographic Message Syntax September 2009
SignedData ::= SEQUENCE {
version CMSVersion,
digestAlgorithms DigestAlgorithmIdentifiers,
encapContentInfo EncapsulatedContentInfo,
certificates [0] IMPLICIT CertificateSet OPTIONAL,
crls [1] IMPLICIT RevocationInfoChoices OPTIONAL,
signerInfos SignerInfos }
DigestAlgorithmIdentifiers ::= SET OF DigestAlgorithmIdentifier
SignerInfos ::= SET OF SignerInfo
EncapsulatedContentInfo ::= SEQUENCE {
eContentType ContentType,
eContent [0] EXPLICIT OCTET STRING OPTIONAL }
SignerInfo ::= SEQUENCE {
version CMSVersion,
sid SignerIdentifier,
digestAlgorithm DigestAlgorithmIdentifier,
signedAttrs [0] IMPLICIT SignedAttributes OPTIONAL,
signatureAlgorithm SignatureAlgorithmIdentifier,
signature SignatureValue,
unsignedAttrs [1] IMPLICIT UnsignedAttributes OPTIONAL }
SignerIdentifier ::= CHOICE {
issuerAndSerialNumber IssuerAndSerialNumber,
subjectKeyIdentifier [0] SubjectKeyIdentifier }
SignedAttributes ::= SET SIZE (1..MAX) OF Attribute
UnsignedAttributes ::= SET SIZE (1..MAX) OF Attribute
Attribute ::= SEQUENCE {
attrType OBJECT IDENTIFIER,
attrValues SET OF AttributeValue }
AttributeValue ::= ANY
SignatureValue ::= OCTET STRING
EnvelopedData ::= SEQUENCE {
version CMSVersion,
originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL,
recipientInfos RecipientInfos,
encryptedContentInfo EncryptedContentInfo,
unprotectedAttrs [1] IMPLICIT UnprotectedAttributes OPTIONAL }
Housley Standards Track [Page 45]
RFC 5652 Cryptographic Message Syntax September 2009
OriginatorInfo ::= SEQUENCE {
certs [0] IMPLICIT CertificateSet OPTIONAL,
crls [1] IMPLICIT RevocationInfoChoices OPTIONAL }
RecipientInfos ::= SET SIZE (1..MAX) OF RecipientInfo
EncryptedContentInfo ::= SEQUENCE {
contentType ContentType,
contentEncryptionAlgorithm ContentEncryptionAlgorithmIdentifier,
encryptedContent [0] IMPLICIT EncryptedContent OPTIONAL }
EncryptedContent ::= OCTET STRING
UnprotectedAttributes ::= SET SIZE (1..MAX) OF Attribute
RecipientInfo ::= CHOICE {
ktri KeyTransRecipientInfo,
kari [1] KeyAgreeRecipientInfo,
kekri [2] KEKRecipientInfo,
pwri [3] PasswordRecipientInfo,
ori [4] OtherRecipientInfo }
EncryptedKey ::= OCTET STRING
KeyTransRecipientInfo ::= SEQUENCE {
version CMSVersion, -- always set to 0 or 2
rid RecipientIdentifier,
keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
encryptedKey EncryptedKey }
RecipientIdentifier ::= CHOICE {
issuerAndSerialNumber IssuerAndSerialNumber,
subjectKeyIdentifier [0] SubjectKeyIdentifier }
KeyAgreeRecipientInfo ::= SEQUENCE {
version CMSVersion, -- always set to 3
originator [0] EXPLICIT OriginatorIdentifierOrKey,
ukm [1] EXPLICIT UserKeyingMaterial OPTIONAL,
keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
recipientEncryptedKeys RecipientEncryptedKeys }
OriginatorIdentifierOrKey ::= CHOICE {
issuerAndSerialNumber IssuerAndSerialNumber,
subjectKeyIdentifier [0] SubjectKeyIdentifier,
originatorKey [1] OriginatorPublicKey }
Housley Standards Track [Page 46]
RFC 5652 Cryptographic Message Syntax September 2009
OriginatorPublicKey ::= SEQUENCE {
algorithm AlgorithmIdentifier,
publicKey BIT STRING }
RecipientEncryptedKeys ::= SEQUENCE OF RecipientEncryptedKey
RecipientEncryptedKey ::= SEQUENCE {
rid KeyAgreeRecipientIdentifier,
encryptedKey EncryptedKey }
KeyAgreeRecipientIdentifier ::= CHOICE {
issuerAndSerialNumber IssuerAndSerialNumber,
rKeyId [0] IMPLICIT RecipientKeyIdentifier }
RecipientKeyIdentifier ::= SEQUENCE {
subjectKeyIdentifier SubjectKeyIdentifier,
date GeneralizedTime OPTIONAL,
other OtherKeyAttribute OPTIONAL }
SubjectKeyIdentifier ::= OCTET STRING
KEKRecipientInfo ::= SEQUENCE {
version CMSVersion, -- always set to 4
kekid KEKIdentifier,
keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
encryptedKey EncryptedKey }
KEKIdentifier ::= SEQUENCE {
keyIdentifier OCTET STRING,
date GeneralizedTime OPTIONAL,
other OtherKeyAttribute OPTIONAL }
PasswordRecipientInfo ::= SEQUENCE {
version CMSVersion, -- always set to 0
keyDerivationAlgorithm [0] KeyDerivationAlgorithmIdentifier
OPTIONAL,
keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
encryptedKey EncryptedKey }
OtherRecipientInfo ::= SEQUENCE {
oriType OBJECT IDENTIFIER,
oriValue ANY DEFINED BY oriType }
DigestedData ::= SEQUENCE {
version CMSVersion,
digestAlgorithm DigestAlgorithmIdentifier,
encapContentInfo EncapsulatedContentInfo,
digest Digest }
Housley Standards Track [Page 47]
RFC 5652 Cryptographic Message Syntax September 2009
Digest ::= OCTET STRING
EncryptedData ::= SEQUENCE {
version CMSVersion,
encryptedContentInfo EncryptedContentInfo,
unprotectedAttrs [1] IMPLICIT UnprotectedAttributes OPTIONAL }
AuthenticatedData ::= SEQUENCE {
version CMSVersion,
originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL,
recipientInfos RecipientInfos,
macAlgorithm MessageAuthenticationCodeAlgorithm,
digestAlgorithm [1] DigestAlgorithmIdentifier OPTIONAL,
encapContentInfo EncapsulatedContentInfo,
authAttrs [2] IMPLICIT AuthAttributes OPTIONAL,
mac MessageAuthenticationCode,
unauthAttrs [3] IMPLICIT UnauthAttributes OPTIONAL }
AuthAttributes ::= SET SIZE (1..MAX) OF Attribute
UnauthAttributes ::= SET SIZE (1..MAX) OF Attribute
MessageAuthenticationCode ::= OCTET STRING
DigestAlgorithmIdentifier ::= AlgorithmIdentifier
SignatureAlgorithmIdentifier ::= AlgorithmIdentifier
KeyEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier
ContentEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier
MessageAuthenticationCodeAlgorithm ::= AlgorithmIdentifier
KeyDerivationAlgorithmIdentifier ::= AlgorithmIdentifier
RevocationInfoChoices ::= SET OF RevocationInfoChoice
RevocationInfoChoice ::= CHOICE {
crl CertificateList,
other [1] IMPLICIT OtherRevocationInfoFormat }
OtherRevocationInfoFormat ::= SEQUENCE {
otherRevInfoFormat OBJECT IDENTIFIER,
otherRevInfo ANY DEFINED BY otherRevInfoFormat }
Housley Standards Track [Page 48]
RFC 5652 Cryptographic Message Syntax September 2009
CertificateChoices ::= CHOICE {
certificate Certificate,
extendedCertificate [0] IMPLICIT ExtendedCertificate, -- Obsolete
v1AttrCert [1] IMPLICIT AttributeCertificateV1, -- Obsolete
v2AttrCert [2] IMPLICIT AttributeCertificateV2,
other [3] IMPLICIT OtherCertificateFormat }
AttributeCertificateV2 ::= AttributeCertificate
OtherCertificateFormat ::= SEQUENCE {
otherCertFormat OBJECT IDENTIFIER,
otherCert ANY DEFINED BY otherCertFormat }
CertificateSet ::= SET OF CertificateChoices
IssuerAndSerialNumber ::= SEQUENCE {
issuer Name,
serialNumber CertificateSerialNumber }
CMSVersion ::= INTEGER { v0(0), v1(1), v2(2), v3(3), v4(4), v5(5) }
UserKeyingMaterial ::= OCTET STRING
OtherKeyAttribute ::= SEQUENCE {
keyAttrId OBJECT IDENTIFIER,
keyAttr ANY DEFINED BY keyAttrId OPTIONAL }
-- Content Type Object Identifiers
id-ct-contentInfo OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) ct(1) 6 }
id-data OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 1 }
id-signedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 2 }
id-envelopedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 3 }
id-digestedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 5 }
id-encryptedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs7(7) 6 }
Housley Standards Track [Page 49]
RFC 5652 Cryptographic Message Syntax September 2009
id-ct-authData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) ct(1) 2 }
-- The CMS Attributes
MessageDigest ::= OCTET STRING
SigningTime ::= Time
Time ::= CHOICE {
utcTime UTCTime,
generalTime GeneralizedTime }
Countersignature ::= SignerInfo
-- Attribute Object Identifiers
id-contentType OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) 3 }
id-messageDigest OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) 4 }
id-signingTime OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) 5 }
id-countersignature OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) 6 }
-- Obsolete Extended Certificate syntax from PKCS #6
ExtendedCertificateOrCertificate ::= CHOICE {
certificate Certificate,
extendedCertificate [0] IMPLICIT ExtendedCertificate }
ExtendedCertificate ::= SEQUENCE {
extendedCertificateInfo ExtendedCertificateInfo,
signatureAlgorithm SignatureAlgorithmIdentifier,
signature Signature }
ExtendedCertificateInfo ::= SEQUENCE {
version CMSVersion,
certificate Certificate,
attributes UnauthAttributes }
Signature ::= BIT STRING
END -- of CryptographicMessageSyntax2004
Housley Standards Track [Page 50]
RFC 5652 Cryptographic Message Syntax September 2009
12.2. Version 1 Attribute Certificate ASN.1 Module
AttributeCertificateVersion1
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) v1AttrCert(15) }
DEFINITIONS EXPLICIT TAGS ::=
BEGIN
-- EXPORTS All
IMPORTS
-- Imports from RFC 5280 [PROFILE], Appendix A.1
AlgorithmIdentifier, Attribute, CertificateSerialNumber,
Extensions, UniqueIdentifier
FROM PKIX1Explicit88
{ iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7)
mod(0) pkix1-explicit(18) }
-- Imports from RFC 5280 [PROFILE], Appendix A.2
GeneralNames
FROM PKIX1Implicit88
{ iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7)
mod(0) pkix1-implicit(19) }
-- Imports from RFC 3281 [ACPROFILE], Appendix B
AttCertValidityPeriod, IssuerSerial
FROM PKIXAttributeCertificate
{ iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7)
mod(0) attribute-cert(12) } ;
-- Definition extracted from X.509-1997 [X.509-97], but
-- different type names are used to avoid collisions.
AttributeCertificateV1 ::= SEQUENCE {
acInfo AttributeCertificateInfoV1,
signatureAlgorithm AlgorithmIdentifier,
signature BIT STRING }
Housley Standards Track [Page 51]
RFC 5652 Cryptographic Message Syntax September 2009
AttributeCertificateInfoV1 ::= SEQUENCE {
version AttCertVersionV1 DEFAULT v1,
subject CHOICE {
baseCertificateID [0] IssuerSerial,
-- associated with a Public Key Certificate
subjectName [1] GeneralNames },
-- associated with a name
issuer GeneralNames,
signature AlgorithmIdentifier,
serialNumber CertificateSerialNumber,
attCertValidityPeriod AttCertValidityPeriod,
attributes SEQUENCE OF Attribute,
issuerUniqueID UniqueIdentifier OPTIONAL,
extensions Extensions OPTIONAL }
AttCertVersionV1 ::= INTEGER { v1(0) }
END -- of AttributeCertificateVersion1
13. References
13.1. Normative References
[ACPROFILE] Farrell, S. and R. Housley, "An Internet Attribute
Certificate Profile for Authorization", RFC 3281, April
2002.
[PROFILE] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation
List (CRL) Profile", RFC 5280, May 2008.
[STDWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[X.208-88] CCITT. Recommendation X.208: Specification of Abstract
Syntax Notation One (ASN.1), 1988.
[X.209-88] CCITT. Recommendation X.209: Specification of Basic
Encoding Rules for Abstract Syntax Notation One
(ASN.1), 1988.
[X.501-88] CCITT. Recommendation X.501: The Directory - Models,
1988.
[X.509-88] CCITT. Recommendation X.509: The Directory -
Authentication Framework, 1988.
Housley Standards Track [Page 52]
RFC 5652 Cryptographic Message Syntax September 2009
[X.509-97] ITU-T. Recommendation X.509: The Directory -
Authentication Framework, 1997.
[X.509-00] ITU-T. Recommendation X.509: The Directory -
Authentication Framework, 2000.
13.2. Informative References
[CMS1] Housley, R., "Cryptographic Message Syntax", RFC 2630,
June 1999.
[CMS2] Housley, R., "Cryptographic Message Syntax (CMS)", RFC
3369, August 2002.
[CMS3] Housley, R., "Cryptographic Message Syntax (CMS)", RFC
3852, July 2004.
[CMSALG] Housley, R., "Cryptographic Message Syntax (CMS)
Algorithms", RFC 3370, August 2002.
[CMSMSIG] Housley, R., "Cryptographic Message Syntax (CMS)
Multiple Signer Clarification", RFC 4853, April 2007.
[DH-X9.42] Rescorla, E., "Diffie-Hellman Key Agreement Method",
RFC 2631, June 1999.
[ESS] Hoffman, P., Ed., "Enhanced Security Services for
S/MIME", RFC 2634, June 1999.
[MSAC] Microsoft Development Network (MSDN) Library,
"Authenticode", April 2004 Release.
[MSG2] Dusse, S., Hoffman, P., Ramsdell, B., Lundblade, L.,
and L. Repka, "S/MIME Version 2 Message Specification",
RFC 2311, March 1998.
[MSG3] Ramsdell, B., Ed., "S/MIME Version 3 Message
Specification", RFC 2633, June 1999.
[MSG3.1] Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.1 Message Specification",
RFC 3851, July 2004.
[NEWPKCS#1] Kaliski, B. and J. Staddon, "PKCS #1: RSA Cryptography
Specifications Version 2.0", RFC 2437, October 1998.
Housley Standards Track [Page 53]
RFC 5652 Cryptographic Message Syntax September 2009
[OCSP] Myers, M., Ankney, R., Malpani, A., Galperin, S., and
C. Adams, "X.509 Internet Public Key Infrastructure
Online Certificate Status Protocol - OCSP", RFC 2560,
June 1999.
[PKCS#1] Kaliski, B., "PKCS #1: RSA Encryption Version 1.5", RFC
2313, March 1998.
[PKCS#6] RSA Laboratories. PKCS #6: Extended-Certificate Syntax
Standard, Version 1.5. November 1993.
[PKCS#7] Kaliski, B., "PKCS #7: Cryptographic Message Syntax
Version 1.5", RFC 2315, March 1998.
[PKCS#9] RSA Laboratories. PKCS #9: Selected Attribute Types,
Version 1.1. November 1993.
[PWRI] Gutmann, P., "Password-based Encryption for CMS", RFC
3211, December 2001.
[RANDOM] Eastlake, D., 3rd, Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC
4086, June 2005.
14. Security Considerations
The Cryptographic Message Syntax provides a method for digitally
signing data, digesting data, encrypting data, and authenticating
data.
Implementations must protect the signer's private key. Compromise of
the signer's private key permits masquerade.
Implementations must protect the key management private key, the
key-encryption key, and the content-encryption key. Compromise of
the key management private key or the key-encryption key may result
in the disclosure of all contents protected with that key.
Similarly, compromise of the content-encryption key may result in
disclosure of the associated encrypted content.
Implementations must protect the key management private key and the
message-authentication key. Compromise of the key management private
key permits masquerade of authenticated data. Similarly, compromise
of the message-authentication key may result in undetectable
modification of the authenticated content.
Housley Standards Track [Page 54]
RFC 5652 Cryptographic Message Syntax September 2009
The key management technique employed to distribute message-
authentication keys must itself provide data origin authentication;
otherwise, the contents are delivered with integrity from an unknown
source. Neither RSA [PKCS#1] [NEWPKCS#1] nor Ephemeral-Static
Diffie-Hellman [DH-X9.42] provide the necessary data origin
authentication. Static-Static Diffie-Hellman [DH-X9.42] does provide
the necessary data origin authentication when both the originator and
recipient public keys are bound to appropriate identities in X.509
certificates.
When more than two parties share the same message-authentication key,
data origin authentication is not provided. Any party that knows the
message-authentication key can compute a valid MAC; therefore, the
contents could originate from any one of the parties.
Implementations must randomly generate content-encryption keys,
message-authentication keys, initialization vectors (IVs), and
padding. Also, the generation of public/private key pairs relies on
random numbers. The use of inadequate pseudo-random number
generators (PRNGs) to generate cryptographic keys can result in
little or no security. An attacker may find it much easier to
reproduce the PRNG environment that produced the keys, searching the
resulting small set of possibilities, rather than brute force
searching the whole key space. The generation of quality random
numbers is difficult. RFC 4086 [RANDOM] offers important guidance in
this area.
When using key-agreement algorithms or previously distributed
symmetric key-encryption keys, a key-encryption key is used to
encrypt the content-encryption key. If the key-encryption and
content-encryption algorithms are different, the effective security
is determined by the weaker of the two algorithms. If, for example,
content is encrypted with Triple-DES using a 168-bit Triple-DES
content-encryption key, and the content-encryption key is wrapped
with RC2 using a 40-bit RC2 key-encryption key, then at most 40 bits
of protection is provided. A trivial search to determine the value
of the 40-bit RC2 key can recover the Triple-DES key, and then the
Triple-DES key can be used to decrypt the content. Therefore,
implementers must ensure that key-encryption algorithms are as strong
or stronger than content-encryption algorithms.
Implementers should be aware that cryptographic algorithms become
weaker with time. As new cryptoanalysis techniques are developed and
computing performance improves, the work factor to break a particular
cryptographic algorithm will be reduced. Therefore, cryptographic
algorithm implementations should be modular, allowing new algorithms
to be readily inserted. That is, implementers should be prepared for
the set of algorithms that must be supported to change over time.
Housley Standards Track [Page 55]
RFC 5652 Cryptographic Message Syntax September 2009
The countersignature unsigned attribute includes a digital signature
that is computed on the content signature value; thus, the
countersigning process need not know the original signed content.
This structure permits implementation efficiency advantages; however,
this structure may also permit the countersigning of an inappropriate
signature value. Therefore, implementations that perform
countersignatures should either verify the original signature value
prior to countersigning it (this verification requires processing of
the original content), or implementations should perform
countersigning in a context that ensures that only appropriate
signature values are countersigned.
15. Acknowledgments
This document is the result of contributions from many professionals.
I appreciate the hard work of all members of the IETF S/MIME Working
Group. I extend a special thanks to Rich Ankney, Simon Blake-Wilson,
Tim Dean, Steve Dusse, Carl Ellison, Peter Gutmann, Bob Jueneman,
Stephen Henson, Paul Hoffman, Scott Hollenbeck, Don Johnson, Burt
Kaliski, John Linn, John Pawling, Blake Ramsdell, Francois Rousseau,
Jim Schaad, Dave Solo, Paul Timmel, and Sean Turner for their efforts
and support.
I thank Tim Polk for his encouragement in advancing this
specification along the standards maturity ladder. In addition, I
thank Jan Vilhuber for the careful reading that resulted in RFC
Errata 1744.
Author's Address
Russell Housley
Vigil Security, LLC
918 Spring Knoll Drive
Herndon, VA 20170
USA
EMail: housley@vigilsec.com
Housley Standards Track [Page 56]